1. The Field of the Invention
The present invention relates to nanocrystals and methods for their fabrication. In particular, the present invention relates to methods for post-synthesis shape and/or size modification of colloidal nanocrystals.
2. The Relevant Technology
Nanocrystals are small crystallites of semiconductors or metals with various shapes (dots, rods, fibers, tetrapods and other geometries) and sizes ranging from 1 to 100 nm. For example, a so-called quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. The most striking feature of semiconductor and metallic nanocrystals is that, in contrast to bulk material, their electronic and optical properties are dependent on particle size and shape and therefore can be continuously controlled over a large range. These unique features make nanocrystals important candidates for advanced applications in areas as diverse as nano-electronics, nano-photonics, solid-state lightning, energy conversion and storage, and health science. For example, nanocrystals are considered key components for next-generation single-photon generation and detection, encryption, micro-lasing and solar energy conversion. In addition, nanocrystals are intensively studied in biological labeling and imaging as well as for targeted drug delivery. For example, nanocrystals are considered to be superior for use as dyes in biological labeling and imaging when compared to conventional molecular dyes because nanocrystal dyes are brighter and they are not generally subject to photo-bleaching.
This wide range of potential applications has sparked research into the development of robust and universal synthesis routes for the fabrication of nanocrystals with adjustable sizes and shapes. Outstanding in these efforts is the seminal work of Murray, Norris and Bawendi in 1993, who reported a relatively simple and robust solution-based synthesis route for the preparation of nearly monodisperse semiconducting cadmium chalcogenide (i.e., CdS, CdSe and CdTe) semiconductor nanocrystal quantum dots. Their technique uses colloidal crystal-nucleation and growth chemistry at a temperature in range of about 200° C. to about 350° C. in the presence of a long alkyl-chain surfactant/solvent system. Example solvents include long-chain alkylphosphines, long-chain alkylphosphine oxides, and long-chain alkenes. However, the solvent/surfactant system used in high-temperature synthesis methods is generally quite expensive and the solvent/surfactant system is generally not reusable from reaction to reaction.
Following the Bawendi Group's discovery, widespread research has been devoted to the synthesis of various types of nanocrystalline materials. While slight modifications of the original Bawendi method in terms of organometallic precursor species and reaction and crystallizations conditions (concentration of reaction components, solvents, growth time, etc.) have resulted in the development of a wealth of nanocrystals with different compositions, sizes, and shapes, it is interesting to note that the typical synthesis conditions are all based on the original high-temperature (e.g., 200-350° C. for cadmium chalcogenide nanocrystals) crystallite nucleation and growth route.
Following on the work of Bawendi and others, widespread research has also been devoted to developing synthesis methods for nanocrystals having different shapes. Currently nanocrystals with shapes other than near-spherical such as rods, fibers, tetrapods and other geometries are formed either during typical colloidal organometallic chemistry-based synthesis of high-quality nanocrystals or by etching methods of synthesized nanocrystals. In the first method the reaction conditions are adjusted so that nanocrystal growth in one or more dimensions is favorable. While this leads to non-spherical shapes, the elongated shapes in one or more directions leads to the partial loss of the quantum confinement and therefore excitons are no longer three-dimensionally confined. The second method uses acid or base etching to modify the shape of synthesized near-spherical nanocrystals. While three-dimensional quantum confinement is retained in this technique, these processes in general require aqueous acid and/or base environments and therefore require several synthesis and post-synthesis steps to retain nanocrystal quality.